Up to now, as an ink-jet ink exhibiting good water resistance, there has been an ink which contains a water soluble dye dispersed or dissolved in a high boiling point solvent, and which contains a water soluble dye dissolved in a volatile solvent. However, a dye is inferior to a pigment in resistance characteristics such as light stability, and therefore ink using a pigment as a coloring agent has been required. But, it is difficult to disperse a pigment stably in an organic solvent, and it is also difficult to ensure stable dispersibility and ejectibility. On the other hand, in cases when ink using a high boiling solvent is applied onto a non-absorptive recording medium, a solvent in the ink is not volatized, results in difficulty of drying via evaporation of the solvent. Accordingly, ink using high boiling solvent cannot be printed on a non-absorptive recording medium.
Generally, ink using a volatile organic solvent can be properly printed onto a non-absorptive recording medium, due to adhesiveness of resin used and volatilization of the solvent. However, such ink, in which the volatile solvent is a major component, is easily dried at the nozzles of a recording head due to volatilization of the solvent, requiring frequent maintenance. Resistance of ink to some solvents may not be sufficient because the ink essentially requires good solubility in a solvent used.
Further, in an on-demand printer using a piezo element, usage of a large amount of a volatile solvent in ink increases frequency of maintenance, and also tends to cause problems of dissolution and swelling of members contacting the ink in the printer. In addition, a volatile solvent has numerous restrictions as a dangerous material under the Fire Defense Law. Consequently, in an on-demand printer using a piezo element, it is necessary to employ ink containing a volatile solvent in a small amount. However, components used in an actinic ray curable ink usually are ones having a relatively high viscosity. Therefore, it has been difficult to design ink having a viscosity capable of being ejected in conventional printers and further having good curability and high stability.
In order to overcome the problems described above, an actinic ray curable composition is disclosed (see for example, Japanese Patent O.P.I. Publication No. 2001-220526) which contains a cationic photopolymerization initiator, an epoxy compound, a compound having an oxetane ring with a hydroxyl group, and a vinyl ether. In the above document, the epoxy compound and the compound having an oxetane ring with a hydroxyl group both are cationically polymerizable compounds having a solubility parameter (sp value) exceeding 10, and the epoxy compound content of the composition is 47 to 63% by weight based on the total weight of cationically polymerizable compound. An extensive study of the above actinic ray curable composition has been studied, and, as a result, it has been proven that ink of this composition has problems in its curability, strength of a cured layer therefrom, ejection stability from nozzles, its adhesion to a substrate, solvent resistance, and water resistance.
Further, an actinic ray curable composition is disclosed (see for example, Japanese Patent O.P.I. Publication No. 2002-188035), which contains 10 to 50% by weight of a compound having an oxirane ring, 50 to 90% by weight of a compound having an oxetane ring and 0 to 40% by weight of a vinyl ether. In the patent above document, the compound having an oxirane ring (epoxy compound) is a cationically polymerizable compound having a solubility parameter (sp value) exceeding 10, the epoxy compound content of the composition is not more than 50% by weight based on the total weight of cationically polymerizable compound, and some of the compound having an oxetane ring have an sp value exceeding 8.0, and others have an sp value of not more than 8.0. An extensive study of the above actinic ray curable composition has been studied, and, as a result, it has been proven that ink of this composition containing particularly a compound having an oxetane ring and having an sp value exceeding 8.0 has problems in its curability, strength of a cured layer therefrom, ejection stability from nozzles, its adhesion to a substrate, solvent resistance, and water resistance.